A method such as described below is normally adopted for the formation of wet etching components. An alkaline-soluble photoresist film is formed to a desired pattern on an iron-based or copper-based metal material, followed by etching a metal portion which is not covered with the aforementioned photoresist using an acidic ferric chloride etching solution or cupric chloride etching solution. In wet etching methods that use an etching solution such as described above, etching proceeds isotropicly from the holes of the photoresist to directly beneath the photoresist film. Consequently, side etching (in which etching occurs directly beneath the photoresist as a result of etching penetrating isotropicly below the photoresist film as well) occurs, which causes difficulty in microprocessing. In addition, the cross-sectional shape of etched processed portions becomes roughly semi-elliptical due to the progression of the aforementioned isotropic etching. For reasons as those described above, even if it is desired to give a high aspect ratio to the etched portions of metal etching products and decrease the pitch between adjacent patterns, problems occurred such that desired ratio and patterns can not be achieved.
An example of a method proposed to solve these problems is described in Japanese Unexamined Patent Application, First Publication No. Hei 1-188700. In this method, after protecting the sides of a primary etched (it may be called as “half-etched”) portion with an insulating protective film, electrolytic etching is conducted again. As a result, the unnecessary portions of the layer subjected to primary etching is removed, and a high-density pattern is formed.
In the aforementioned document, an insulating protective film is formed over the entire surface of a primary etched portion (hole portions), after which the bottom corresponding to the deep portion of the insulating protective film of the primary etched portion is dissolved simply by spraying with an acidic solution. Consequently, if variations occur in the flow of spray of the acidic solution within the surface of the metal plate, dimensional variations occur easily in the protective film within the primary etched portion. This results in a very serious problem particularly in the case of formation of high-definition patterns having a narrow pitch between adjacent patterns. In addition, in the case of high-definition patterns, since the amount of side etching is small and overhangs (overhanging portions formed at the corners of the surface and etched portion) formed by primary etching are minute, the problem occurs in which the insulating protective film is completely removed. In addition, in the case of forming a pattern which provides a large amount of etching in the vertical direction and the deep holes are formed by primary etching, removal of only deep bottom portion of the insulating protective film cannot be achieved by the aforementioned method. As a result, contrary to what is desired, etching proceeds over the entire surface during second etching.
In addition, a method is also adopted like that described in Japanese Examined Patent Application, Second Publication No. S 58-15537. In this method, the overhang of a photoresist formed by primary etching is used as a photomask, and a positive photoresist coated again on the primary etched surface is exposed and developed. The unnecessary portion of the layer subjected to etching is removed, and the positive photoresist remains on the sidewall formed by primary etching, resulting in the formation of a high-density pattern.
In the aforementioned method, if chipping or running occurs even in a portion of the photoresist overhang formed by primary etching, that deformation ends up being transferred to the shape of the resist of the second etching. Accordingly, a sharp shape is unable to be obtained, thus making this unsuitable for forming a fine pattern. This presents a serious problem particularly in the case of formation of high-definition patterns having a narrow pitch between adjacent patterns. In addition, in this method, it is difficult to uniformly coat the second photoresist in the holes formed by primary etching. Thus this method is particularly unable to accommodate to formation of high-definition patterns and patterns having a large amount of etching in which deep holes are formed by primary etching.
Moreover, an other example of a method that attempts to solve the aforementioned problems is described in Japanese Examined Patent Application, Second Publication No. S 62-37713. In this method, after filling a low molecular weight solvent-based adhesive into a surface formed by primary etching, the adhesive is allowed to dry. As a result, the overhangs of the softened resist are forcibly adhered to the primary etched surface, and the unnecessary portion of the etched layer is removed to form a high-density pattern.
In this method, there is the problem such that the resist components of the primary etching resist is dissolved by the adhesive and sporadically remained in the bottom of the primary etched portion to cause sporadic treatment in next etching. There is also the problem of the adhesive causing swelling of the resist overhangs, and a sharp shape of the overhangs is not reproduced in the drying step. These problems cause variations in the etching dimensions in the surface. In addition, since the resist overhangs of primary etching are minute in the case of high-definition patterns, the effect of adhering the overhangs is unable to be obtained and merely the adhesive comprising adhesion liquid is completely evaporated. In addition, even when attempting to obtain holes having a high aspect ratio, since side etching can only be prevented at the overhangs, this method is incompatible with patterns having deep primary etching holes that have a large amount of etching.
As has been described above, since it is difficult to carry out second etching processing both stably and with high accuracy using technology like that described in the aforementioned patent documents, and since half-etched holes of the primary etching resist is used to form the second etching resist, it is difficult to carry out processing of a different shape than primary etching in second etching and beyond.